Protecting Plants From Frost Damage

Before you do anything else, look at the average minimum temperature for your area as listed on the plant hardiness zone map
for your area.

Compare that to the minimum temperature requirements of
your plant. You can often find that on the plant's label, online, or in
plant information books.

It may be that your plant (like the kale in the photo) doesn't need protecting at all!

The bigger the difference between
your plant's minimum temperature requirements and your area's hardiness
zone, however, the more protecting you'll need to do in order to keep your plant
alive during the winter.

How do plants survive the winter?

Some hardy plants

have thick bark,

have sap that acts
like "anti-freeze" in protecting the plant's cells from freezing,

drop
tender leaves and go into dormancy,

and so on.

But the main strategies
plants use to survive the winter have to do with protecting their roots and sending those roots below the frost line. As long as the
roots remain unfrozen, even if the upper part dies back,
the plant will return when the weather warms again.

So you want to plant early in the year to give your plant the best chance to get its roots deep into the earth before cold weather comes.

Also, put your plant in the warmest area of your yard.

This will usually be in full sun, next to a south-facing (or
north-facing, for those of you in the Southern Hemisphere) wall, with
wind protection to prevent wind chill.

Brick or stone walls are best,
because they will retain the heat of the day and release it overnight.

You can potentially get a good five to ten
degrees Fahrenheit (2.8 to 5 degrees C) rise in the temperature
surrounding your plants by using these strategies -- up to a whole zone
improvement!

Use raised beds in areas with a lot of snow. The ground will drain better and warm faster in a raised bed than in a regular bed, especially if you use dark-colored material to build the raised bed from.

If your plants can tolerate being planted deeply, do so.
Some plants are very sensitive to how deeply they're planted, but some
don't seem to care. Some, like tomatoes, prefer to be planted deeply.
The soil temperature is more stable the farther down you go, so if you can plant more deeply, it's a good idea.

Mulch, mulch, mulch!

Mulching puts a layer of insulation between your
plants roots and the freezing weather. The deeper the layer, the warmer
your plant will be, just as if you put a thick coat on versus a thin
sweater. This can go a long way in protecting plants from frost and
freezing.

Dark-colored wood/compost mulch and rock/gravel
mulch both retain heat -- which you use will depend on your plants and what they need. Rock and gravel are better if you need good drainage, while dark
wood or compost mulch will add nutrients later on.

Both soak up the
sun's heat and transfer it to the soil, warming it and protecting plants
from frost.

Row covers are rolls of special fabric used to cover your plants. While mulch covers the roots,
floating row covers
protect the tops of your plants from the wind yet allow light to go through.

You can either cover a whole row
of plants at once, or cut the row cover to fit just one plant. Use
ground staples
to secure your row cover material from being blown by the wind.

This is
an easy and simple method of protecting plants from frost that can be
used along with other methods as well.

Usually row covers are temperature rated. I've found
that doubling or tripling the row covers works just as well in
protecting plants from frost as buying a whole new roll of
higher-rated stuff, but I would err on the side of caution for
protecting expensive plants or perennials going through their first winter.

Between mulching and row covers, you can get
another zone or two worth of frost protection. If your plants are
covered with snow, this may be all you need, because snow is a great
insulator.

Check plants under the row covers during the winter to see if you
need anything more as the temperature drops. Signs to look for are leaf
damage and wilting.

Sometimes when the row cover touches the plant,
it allows the cold to penetrate because there's not that bubble of
warmer air for insulation. For a plant that's on the edge of freezing,
that can be enough to cause damage.

If you find that happening,
constructing a hoophouse type frame to raise the row covers above your plants can help.

If you live where the temperature drops below 0
degrees F (-17 degrees C), you might want to go further in protecting
plants from frost.

There are two methods you might use:

Further insulation such as burlap
and leaves, good for insulation of young trees, rose bushes, semi-hardy
fruit shrubs, and so on. Generally this is done before the freezing
weather arrives. Wrap the tree loosely with burlap, then fill
the space between the burlap and the tree with leaves. This offers
protection to the roots and base of the tree.

Covering with an impermeable material such as plastic or glass.

Protecting plants from frost using plastic and glass

The cloche is a bell-shaped glass cover -- used in
gardens where hail isn't likely -- to overwinter delicate plants
outdoors or start seedlings early.

These can be very useful in areas
with relatively mild weather yet short growing seasons.

Some people construct plastic cloches out of leftover bottles to help in protecting their tender seedlings early in spring. If you find a source of attractive bottles, this can be a good alternative to glass cloches in areas that do get hail.

A cold frame
is generally a wood box covered with glass on top. These is basically a
mini green house.

Cold frames are a good alternative for people with small
yards, useful to start seeds in winter for later planting, to overwinter
small potted plants, and to extend the season for plants that
tolerate shorter day lengths but can't tolerate freezing.

In most areas, the heat of the sun warms a cold
frame sufficiently, but in extremely cold areas, you may need to add a
heat mat underneath your cold frame to maintain its temperature.

Fruit and nut trees are a particular challenge
to keep warm. The critical time for setting fruit is when a tree blooms.
If the temperature drops below freezing at that time, then the tree
will drop its flowers and you won't get any fruit. This can be a real
issue in high latitudes, where the last frost date may be in mid-summer.

One tip I learned while researching this question
is to string incandescent "Christmas" lights in blooming fruit trees
during freezing weather. These
bulbs warm trees quite nicely!

There are many types of greenhouses
(which is a generic term meaning a building used to grow plants in).
These provide the most protection for your plants.

As with the cold
frame, most stand-alone greenhouses are heated by the sun alone, but
greenhouses in areas where there is very limited daylight (especially
the extreme north) or frequent cloud cover, under floor heating or a
green house heater may be necessary.

Hoophouses
come in many sizes, from a few feet high to huge commercial green
houses the size of an airline hangar (also called high tunnels). These are just a row of hoops set into a rigid base and covered in
row cover, canvas, or most commonly, plastic sheeting. These are easy to
set up, are safe for areas that have hail, and work well in protecting
plants from frost.

The disadvantage is that the cover needs to be
replaced every few years due to the sun's effect on it. Also, a hoop
house needs to be securely fastened to the ground in areas with high
winds.

Conservatories
(sometimes called patio rooms, sunrooms, solariums, a hothouse, or
Florida room) are rooms with glass walls (and often a glass roof as
well), attached to the house on one side. This arrangement allows you to
take care of your plants without having to go outside.

You can turn any room in your home with multiple
sunny windows into a sunroom for your potted plants and seedlings. I
use a corner of my garage in that way.

The biggest challenge with plastic and glass
frost protection methods is overheating. The temperature should be
monitored and a way to ventilate should be added (a temperature sensor
that opens a window at the top of the greenhouse when the temperature
rises too high can be used)